This invention relates to aircraft hydraulic fluids containing phosphate ester base stocks as functional fluids and more particularly to the use of certain alkoxylated polyetheramines as additives in such functional fluids for providing improvements in properties such as density, viscosity, lubricity and hydrolytic stability.
Functional fluids are used in a wide variety of applications. For example, they are used as electronic coolants, power transmission and hydraulic fluids, and refrigeration equipment fluids to mention a few. Hydraulic fluids that are to be used in aircraft applications must meet certain performance criteria among which are thermal stability, fire resistance, low susceptibility to viscosity changes over a wide range of temperatures, hydrolytic stability and good lubricity.
In currently available commercial aircraft hydraulic fluids, phosphate esters are the most commonly used base stocks of which tributyl phosphate, isopropylated triphenyl phosphates, n-butyl diphenyl phosphate, and di-n-butyl phenyl phosphate are widely used components.
In use aircraft hydraulic fluids can become contaminated by water; and, as is well known water will cause hydrolysis of phosphate esters to produce partial esters of phosphoric acid. Consequently, phosphate ester based aircraft fluids are formulated to contain an acid scavenger to neutralize any acid or acids produced.
Aircraft components generally are exposed to a very wide temperature range. Therefore, viscosity index improvers are added to aircraft hydraulic fluids to limit the effect of temperature on the viscosity of the fluid composition.
Other additives that are typically used in functional fluids include erosion inhibitors, rust inhibitors, and defoamers.
Although fluid formulators have been successful in developing functional fluid compositions that provide satisfactory properties for aircraft applications, there remains a need for functional fluids that display improved viscosity, lower density, good lubricity and improved hydrolytic stability.
Broadly stated, the functional fluid of the present invention comprises a major amount of an organo phosphate ester base stock; from about 0.5 to about 10 wt % of an alkoxylated polyetheramine having the formula 
where
R1 is a C1 to C24 hydrocarbyl group
R2, R3 and R4 are independently selected from groups represented by 
xe2x80x83where R5 is hydrogen, methyl or ethyl;
Z is a straight or branched alkylene of from 3 to 4 carbon atoms;
x is 1 to 15; and y is 1 to 15. The functional fluid also contains from about 4 to about 20 wt % based on the total weight of fluid of auxiliary additives selected from the groups consisting of antioxidants, viscosity index improvers, rust inhibitors, erosion inhibitors, acid scavengers, defoamers and mixtures thereof.
The functional fluid is particularly useful as an aircraft hydraulic fluid.
The functional fluids of the present invention includes a major amount of an organic phosphate ester base stock. Typical organo phosphate ester base stocks suitable for use in the present invention include esters selected from triaryl phosphates, trialkyl phosphates, dialkyl aryl phosphates, alkyl diaryl phosphates, and alkylated triaryl phosphates that contain from 3 to 8 and preferably from 4 to 5 carbon atoms in the alkyl groups and mixtures thereof. Examples of the foregoing esters include tri-n-butyl phosphate, tri-isobutyl phosphate, n-butyl di-isobutyl phosphate, di-isobutyl n-butyl phosphate, n-butyl diphenyl phosphate, isobutyl diphenyl phosphate, di-n-butyl phenyl phosphate, di-isobutyl penyl phosphate, tri-n-pentyl phosphate, tri-isopentyl phosphate, triphenyl phosphate, isopropylated triphenyl phosphates and butylated triphenyl phosphates. Preferably, the trialkyl phosphate esters are those of tri-n-butyl phosphate, tri-isobutyl phosphate and mixtures thereof.
The amount of each type of phosphate ester in the functional fluid will vary depending upon the specific properties required for the fluid. An ester base stock for an aircraft hydraulic fluid generally will comprise:
(1) from about 10 wt % to 100 wt % and preferably from 20 wt % to 90 wt % of a trialkyl phosphate;
(2) from 0 wt % to about 15 w % and preferably from 0 wt % to about 50 wt % of a dialkyl ayl phosphate;
(3) from 0 wt % to about 30 wt % and preferably from 0 wt % to about 10 wt % of an alkyldiaryl phosphate;
(4) from 0 wt % to about 20 wt % and preferably from 0 wt % to about 15 wt % of a triaryl phosphate.
The functional fluids of the present invention include an alkoxylated polyetheramine having the formula 
where
R1 is a C1 to C24 hydrocarbyl group;
R2, R3 and R4 are independently selected from groups represented by 
xe2x80x83where
R5 is hydrogen, methyl or ethyl;
Z is a straight or branched chain divalent alkylene of from 3 to 4 carbon atoms;
x is 1 to 15; and y is 1 to 15; and y is 1 to 15.
With respect to R1 in the above formula suitable hydrocarbyl groups include linear or branched alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl isobutyl, isobutyl and the like and aryl alkyl groups such as phenyl nonyl and alkyl aryl groups such as pentyl phenyl. In the practice of the present invention R1 preferably is a branched alkyl group of from about 6 to 21 carbon atoms.
Also in the present invention it is preferred that x is in the range of 8 to 15 and y in the range of 1 to 2.
The alkoxylated polyether amines are added to the phosphate ester base stock in amounts ranging from about 0.5 to about 10 wt % based on the total weight of fluid.
Surprisingly, it has been found that the inclusion of alkoxylated polyetheramines in phosphate ester base stocks provides for improved fluid properties such as enhanced low temperature viscosity, hydrolytic stability, lubricity and the like.
In addition to containing the major amount of a phosphate ester base stock and the alkoxylated polyetheramines, the functional fluids of the present invention also include from about 4 wt % to about 20 wt % of auxiliary additives selected from the group consisting essentially of antioxidants, acid scavengers, viscosity index (VI) improvers, rust inhibitors, erosion inhibitors and defoamers.
Useful antioxidants include trialkyphenols, polyphenols and di (alkyl phenyl)amines. These include bis(3,5-di-tert-butyl4-hydroxylphenyl)methane and 1,3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4-hydroxyphenyl)benzene sold under the trade names Hitec(copyright)702 and Ethanox(copyright)330, respectively by Ethyl Corporation. Other examples of antioxidants include tetrakis (methylene[3,5-di-tert-butyl-4-hydrocinnamate)methane sold under the trade name Irganox(copyright)1010 by Ciba-Geigy and di (n-octylphenyl) amine sold under the trade name Vanlube(copyright)81 by Vanderbilt. Typically the antioxidant will be used in the range of from about 0.1 wt % to about 2 wt % based on the total weight of the fluid.
Suitable acid scavengers include epoxy compounds such as expoxycyclohexane alkyl carboxylates, an example of which is 3.4-epoxycyclohexane-2-ethylhexyl carboxylate described in U.S. Pat. No. 3,723,320. Typically the acid scavenger will be used in an amount ranging from about 1 to about 10 wt % based on the total weight of the functional fluid.
Erosion inhibitors that are suitable for use in the compositions of the present invention include alkali metal salts of perfluoroalkyl sulfonic acids such as potassium perfluorooctyl sulfonate sold under the trade name FC(copyright)98 and available from 3M Company. Typically the erosion inhibitor will be present in an amount ranging from about 0.01 wt % to about 0.1 wt % based on the total weight of the functional fluid.
Suitable VI improver additives include polyacrylate esters having a number average molecular weight in the range of about 50,000 to 100,000. Such poly alkyl methacrylates sold as PA(copyright)-7570, PA(copyright)-6703, PA(copyright)-6744, and PA(copyright)-6961-PMN by Rhom and Haas Company. Typically the VI improver will be used in an amount ranging from about 3 wt % to about 10 wt % based on the total weight of the functional fluid.
The functional fluids described herein optionally contain further conventional additions such as metal corrosion inhibitors, anti-foaming agents, dyes, etc.